A wolf's howl is one of the most iconic sounds of nature, yet biologists aren't sure why the animals do it. They're not even sure if wolves howl voluntarily or if it's some sort of reflex, perhaps caused by stress. Now, scientists working with captive North American timber wolves in Austria report that they've solved part of the mystery.

Almost 50 years ago, wildlife biologists suggested that a wolf's howls were a way of re-establishing contact with other pack members after the animals became separated, which often happens during hunts. Yet, observers of captive wolves have also noted that the pattern of howls differs depending on the size of the pack and whether the dominant, breeding wolf is present, suggesting that the calls are not necessarily automatic responses.

'Variations in their howling'

Friederike Range, a cognitive ethologist at the University of Veterinary Medicine in Vienna, and her colleagues have hand-raised nine wolves since 2008 at the Wolf Science Center in Ernstbrunn. Although the center's wolves don't hunt, they do howl differently in different situations, Range said. "So we also wanted to understand these variations in their howling."

• In almost all cases, the pack began to howl within the first 20 minutes after a member was led away on a walk, Range said. But the one out for a stroll usually did not return the call.

• Those left behind howled in 26 of the 27 walking trials, but only twice in the control trials.

• Individual wolves also howled more when the wolf that was led away was his or her preferred pal — which means that the wolves aren't simply howling because others are. "It's not a contagious response," Range said. "Social relationships are very important to them, and the howling patterns reflect that."

• Overall, they did most of their yodeling when the dominant member went for a walk.

'It's strategic, not emotional'

Thinking that the stress of separation likely triggered the wolves' howls, the scientists tested the animals' levels of the stress hormone cortisol. Their cortisol levels spiked when the dominant animal was taken for a walk, but not when their preferred partner was led away. Despite their numerous howls in the latter situation, they were apparently not stressed. And that means that the wolves' howls aren't always a physiological stress response, but can be more voluntary and driven by social factors, the team reported online in Current Biology.

"It's strategic, not emotional," Range said. "They're trying to contact individuals that are important to them and reform the pack. And they have some control over how much they howl."

"The paper provides the first experimental evidence that the main reason [for howling] is to help the pack assemble after a long hunt," said Dave Mech, a wolf biologist at the University of Minnesota, Twin Cities. He proposed the notion in 1966 after witnessing a pack of 15 wolves hunting. At the end of that hunt, the wolves were widely dispersed, he says, but "after howling, the pack was able to assemble again."

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